Such a level sensor is known from U.S. Pat. Nos. 5,399,979, 6,101,873 or 6,761,067.
For measuring the filling level of a measuring medium, such as a fluid or bulk material, in a vessel the known sensor has an elongated, for example a rod-shaped, probe which extends vertically over the fillable vessel height and dips into the measuring medium. The probe is longitudinally segmented into a plurality of segments, each segment comprising an electrode and each electrode forming an electrical capacitor to a common ground electrode, such as the conductive vessel wall or a separate elongated electrode, which can be arranged inside or outside the vessel or can be formed directly on the probe.
The level measurement, which also includes detecting multiple medium layers, such as water and oil, and interfaces between them, is performed by successively and separately measuring the capacities of the individual capacitors.
For this purpose, the electrodes or capacitors can be successively and separately connected to the input of a measuring circuit by means of a plurality of controllable switches. The measuring circuit then generates successively a plurality of measuring signals in dependence on the capacities of the individual capacitors. If the switches are located in the head portion of the probe near to the measuring circuit, the probe comprises a plurality of signal lines between the electrodes and the switches, the number of the signal lines corresponding to the number of the electrodes or switches. If the switches are arranged in the associated segments of the probe only one signal line between the switches and the measuring circuit but a plurality of control lines for controlling the switches are needed, the number of the control lines corresponding to the number of the electrodes or switches. U.S. Pat. No. 4,003,259 discloses an alternative embodiment in which the switches are reed contacts which can be successively opened and closed by a magnet movable along the probe.
Alternatively to the aforementioned solutions, a plurality of controllable measuring circuits are individually arranged in the segments of the probe, each measuring circuit being connected to an associated one of said electrodes. A control means successively and separately activates the measuring circuits to generate measuring signals in dependence on the capacity of the respective capacitors. The probe thus comprises a corresponding plurality of control lines between the measuring circuits and the control means.
The or each measuring circuit may comprise a signal generator for applying an electrical signal to the respective capacitor and a receiver for measuring the voltage across or the current through the capacitor. Alternatively, the or each measuring circuit comprises an oscillator wherein the respective capacitor forms a frequency-determining element of said oscillator.
JP 60-213822 discloses a capacitive level sensor in which the electrodes are connected to the clock input of associated flip-flops which form a shift register. Therefore, the clock signal, which is applied to the clock inputs via resistors, experiences a delay dependent on the capacity of respective the electrode. The Q outputs of the flip flops are then processed to determine the level. The known level sensor only detects a change in capacity and thus the level to be determined. The level sensor, however, is not capable of measuring the capacities of the individual capacitors.
The disadvantage of the known level sensor is, that it comprises a plurality of signal or control lines extending through the length of the probe, thus complicating the design and the manufacture of the sensor, especially when the number of electrodes is large or when different sensor types with different numbers of electrodes are to be manufactured.